Gypsy moth defoliation (photo courtesy USDA Forest Service)
Understanding the drivers of outbreak cycles and synchrony is critical to developing effective management plans to assuage impact. Periodic cycles are found in populations of a broad range of taxa, particularly in forest insects, and are usually attributed to trophic interactions. The gypsy moth (Lymantria dispar L.) is a Eurasian forest defoliator that is invasive in North America. My co-authors and I found that gypsy moth outbreaks in North America exhibit two dominant periodicities of 4-5 years and 8-10 years. We found geographic variation in periodicity that was associated with forest type. The 5-year periodicity was stronger in the most susceptible forest types, xeric sites dominated by oak species (Johnson et al. 2006 Ecography). A post-doctoral researcher in my laboratory, Dr. Kyle Haynes, showed that the bimodal periodicity is largely the result of harmonic population oscillations and not an artifact of aggregating dynamics over large spatial scales (Haynes et al. 2009 Oecologia).
Synchronous population dynamics of forest insects increases ecological and economic impacts because it dilutes the regulating effects of natural enemies, reduces the landscapes ability to buffer the disturbance, exacerbates the economic burden on individual stakeholders, and overwhelms the logistical abilities of managers to suppress populations. Gypsy moth outbreak cycles are synchronous over hundreds of kilometers (Johnson et al. 2005 Journal of Animal Ecology), but the underlying mechanism for the synchrony is not well resolved. I am interested in how synchrony can propagate across trophic interactions. Densities of the main predator of gypsy moth, the white-footed mouse, and its main winter food source, red oak acorns, fluctuate synchronously over distances comparable to that of the gypsy moth. In my laboratory, we used a consumer-resource model of acorn, mouse, gypsy moth, and gypsy moth virus dynamics to support the hypothesis that asynchrony in acorn masting propagating across trophic levels would override any synchronizing effects acting directly on gypsy moth populations (Haynes et al. 2009 Ecology). To my knowledge, this is the first study to demonstrate that synchrony in one species was necessary to maintain synchrony in a trophically-linked species.In an earlier study, I demonstrated how synchronous outbreaks can catalyze pulses in gypsy moth invasion rate in the presence of Allee effects (depressed population growth rate at low density) (Johnson et al. 2006 Nature). This study suggests that suppression of outbreaks near the invasion front, or disruption of synchronous outbreaks, may reduce spread of the gypsy moth across the US.